• Korean Journal of Materials Research
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• v.12 no.1
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• pp.27-35
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• 2002

In laser materials processing, localized heating, melting and evaporation caused by focused laser radiation forms a vapor on the material surface. The plume is generally an unstable entity, fluctuating according to its own dynamics. The beam is refracted and absorbed as it traverses the plume, thus modifying its power density on the surface of the condensed phases. This modifies material evaporation and optical properties of the plume. A laser-produced plasma plume simulation is completed using axisymmetric, high-temperature gas dynamic model including the laser radiation power absorption, refraction, and reflection. The physical properties and velocity profiles are verified using the published experimental and numerical results. The simulation results provide the effect of plasma plume fluctuations on the laser power density and quantitative beam radius changes on the material surface. It is proved that beam absorption, reflection and defocusing effects through the plume are essential to obtain appropriate mathematical simulation results. It is also found that absorption of the beam in the plume has much less direct effect on the beam power density at the material surface than defocusing does and helium gas is more efficient in reducing the beam refraction and absorption effect compared to argon gas for common laser materials processing.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 1998.04a
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• pp.292-299
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• 1998

Case hardening behavior of tool steel(SK5) was investigated after YAG laser irradiation. In the case of beam passes, martensite formed in the melt zone and in former pearlite regions of the austenization zone exhibited very high Vickers Hardness values. The molten depth and radius, micro structure, hardness were investigated as a function of defocusing distance, pulse width, and power density.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.9 no.2
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• pp.66-71
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• 2000

Laser induced surface hardening of Tool steel(STC5) can be achieved either with or without surface melting. In trans-formation hardening as the surface is heated to a temperature below its melting point and is rapidly cooled solidified microstructures are usually much finer and stronger than those of the base matals. For this reason surface modification of tool steel by YAG laser irradiation has been studied as a function of processing parameters such as power density pulse width defocusing distance and molten depth. The high energy density changes and refines the microstructure of the near surface layer. In the case of beam passes martensite formed in the melt zone exhibited very high vickers hardness values. Molten depth and width depend on defocusing distance and energy of black color painting is more absorptive than other color painting.

• Proceedings of the KOSOMBE Conference
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• v.1998 no.11
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• pp.219-220
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• 1998

Although a single-element synthetic aperture system can produce high-resolution beam profile, it is not a highly practical system because of its low signal to noise ratio against conventional system's. A multi-element synthetic aperture processing has been proposed with defocusing in this paper. A multi element subaperture defocused to emulate a single element spatial response with high acoustic power. The results have higher signal to noise and better contrast resolution than conventional synthetic aperture method.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.3
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• pp.315-321
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• 2001

Aerodynamic lenses are widely used in generating particle beams of high density and small diameter, but analytical or modeling studies are limited only in the free molecular regime. In this study, it is shown that generating particle beam is also possible in atmospheric pressure range, and the mechanism of generating particle beam using an orifice is analysed into three different parts : fluid dynamic contraction, diffusional defocusing, and inertial focusing. In laminar flow conditions, the diffusional defocusing effect can be neglected, and the effects of inertial focusing can be expressed in terms of the orifice size and Stokes number. Numerical experiments are done for two different orifices, d/D=1/5 and 1/10 and particle diameter d(sub)p=1-10 ㎛. The results for two different orifices can be made into a single curve when a modified Stokes number is used. The inertial focusing effect diminishes when the modified Stokes number becomes smaller than 10(sup)-2.

• Journal of Welding and Joining
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• v.25 no.3
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• pp.85-91
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• 2007

The conventional study on the laser surface transformation hardening has been carried out with a beam of the specified shape and uniform power-intensity distribution in order to ensure the uniformity of the hardening depth. Two types of beams - the circular gaussian beam and rectangular beam of the uniform power-intensity distribution were used in this study. we were supposed to optimize the process parameters and to compare the hardening results with two optics respectively. As a result, the hardness distribution of the hardened zone was similar in both cases and the hardened phase by the rectangular beam was denser than that by the circular gaussian beam.

• Laser Solutions
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• v.6 no.2
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• pp.1-7
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• 2003

Defocused CO$_2$ laser beam irradiation strengthening of 60kgf/$\textrm{mm}^2$ grade steel sheet was investigated to obtain the tensile strength similar to that of fully penetrated one by the focused beam. The melted line width by the defocused beam was 3∼4 times larger compared to that of the focused beam. However, the increase of tensile strength with 1 line irradiation by the defocused beam was similar to that of 2~3 lines by the focused beam. The increase(37.6%) of bending strength with 1 line by the defocused beam was higher than the increase(12.9%) of tensile strength. Also, the effect of cooling gas to strengthening was observed.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1998.03a
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• pp.53-58
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• 1998

Case hardening of tool steel(SK5) was investigated after YAG laser irradiation. In the case of beam passes, martensite formed in the melt zone and in former pearlite regions of the austenization zone exhibited very high Vickers Hardness values. The molten depth and radius, micro structure, hardness were investigated as a function of defocusing distance, pulse width, and power density.

• Proceedings of the Optical Society of Korea Conference
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• 2000.02a
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• pp.160-161
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• 2000

The 3rd order nonlinear properties show Optical bleaching (Saturation) and Reverse saturation in absorption aspect, whereas Self-focusing[3][4] and Self-defocusing in refraction aspect. Optical bleaching and Self-focusing phenomena of those properties in particular can be useful to make the optical beam spot size smaller for application on the higher optical storage density. (omitted)

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1995.10a
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• pp.921-924
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• 1995

Case hardening behavior of carbon tool steel(SK5) was investigated after pulsed YAG laser irradiation. In the case od beam passes,martensite formed in the melt zone and in former pearlite regions of the austenitization zone exhibits vary high Vickers hardness values. The molten depth and width decrease as the beam power density increase. The influence of depth and width of color painted specimen was also investigated. The molten zone of the black painted specimen was the largest. The were loss of the black painted specimen was smaller than any other painted or raw material.